1. Field of the Invention
The present invention relates to selected phenolic resins made up of a condensation product of an aldehyde comprising either a haloacetaldehyde source or a mixture of a haloacetoaldehyde source and a formaldehyde source with selected phenolic monomers. Furthermore, the present invention relates to light-sensitive compositions useful as positive-working photoresist compositions, particularly, those containing these phenolic resins and o-quinonediazide photosensitizers. Still further, the present invention also relates to substrates coated with these light-sensitive compositions as well as the process of coating, imaging and developing these light-sensitive mixtures on these substrates.
2. Description of Related Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or coppe plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions--negative-working and positive-working. Both negative-working and positive-working compositions are generally made up of a resin and a photoactive compound dissolved in a suitable casting solvent. Additives may be added for specific functions. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution, plasma gases or the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Preferred positive-working photoresists today generally involve novolak-related resins and o-quinonediazide photoactive compounds dissolved in a suitable solvent.
Photoresist resolution is defined as the smallest feature which the resist composition can be consistently transferred from the photomask to the substrate with a high degree of image edge acuity after exposure and development under conditions of reasonable manufacturing variation (such as variations in resist application, exposure energy, focus variation, development variation, and the like). In other words, resolution is the smallest feature which a photoresist user has the practical ability to produce clean images. In many manufacturing applications today, resist resolution on the order of one micron or less are necessary.
In addition, it is generally desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. Moreover, it is also desirable that the photoresists have high thermal flow stability, high alkaline solubility and little or no scumming in the spaces between the developed lines.
One drawback with positive-working photoresists known heretofore is their limited resistance to thermal image deformation during heat generating processes and to dry etching steps. This is becoming an increasing problem because modern processing techniques in semiconductor manufacture (e.g. plasma etching, ion bombardment) require photoresist images which have higher image deformation temperatures (e.g. 140.degree. C.-200.degree. C.). Addressing these problems, photoresist manufacturers have developed options such as higher molecular weight novolak resins, more thermal-etch-resistant polymer species and silicon-containing resins. Processing techniques, such as alternate postbake cycles and deep-UV hardening have been developed to enhance resist image thermal stability. Past efforts to increase thermal stability (e.g. increased molecular weight of the resin) generally caused significant decrease in other desirable properties (e.g. decreased photospeed, diminished adhesion, or reduced contrast, poorer developer dissolution rates, or combinations thereof).
Accordingly, there is a need for improved novolaks useful in positive-working photoresist formulations which are capable of producing images that are resistant to thermal deformation at temperatures of about 140.degree. to 200.degree. C. or higher while maintaining the other desired properties (e.g. alkaline developer dissolution rates, low scumming and good resolution) at suitable levels. The present invention is believed to be an answer to that need.